1. Field of the Invention
The present invention relates to a system and method for efficiently performing two-way ranging to determine the location of a wireless node, such as a user terminal, in a communications network. More particularly, the present invention relates to a system and method for reducing the number of transmissions required between a source node and reference nodes to determine the geographic location of the source node, and for reducing the number of transmissions required for the source node to retrieve data from any of the reference nodes.
2. Description of the Related Art
Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) is a well-known protocol specified in the IEEE802.11 standard for wireless local area networks (LANs) which enables wireless communications devices to communicate with each other. CSMA/CA involves an initial handshake of a Request-to-Send (RTS) message followed by a Clear-to-Send (CTS) message exchanged between a source node and a destination node prior to sending an information message (e.g., a message containing audio, multimedia or data information).
Specifically, the source node transmits an RTS message to the intended destination node. If the intended destination node wishes to receive the message and believes that the channel is available (i.e., free of other traffic), the destination node responds with a CTS message. The receipt of the CTS message by the source node permits the transmission of the information message (MSG) which is typically followed by an Acknowledgment (ACK) message from the destination node when reception of the information message is successful. All other nodes within range of the CTS message mark the channel as busy for the duration of the message transfer. Provided that all nodes receive every CTS message, the protocol works well.
An example of a network employing CSMA/CA techniques is described in U.S. patent application Ser. No. 09/705,588, referenced above. In this type of network, a plurality of nodes communicate with each other using plural, shared parallel data channels and a separate reservation channel. As used herein, the term node refers to a communication device operating in a network of communication devices. The node may be a mobile communication device, such as a radio or wireless telephone, or the node may be stationary or fixed in a particular location. Also, as used herein, the term channel refers to a communication path between nodes, and different channels can exist on separate communication media or on a common communication medium, with individual channels being separated by any suitable means, such as time, frequency, or encoding.
Increased throughput is achieved by transferring messages over the multiple parallel data channels. One channel is dedicated for use as the reservation channel, and channel access is allocated on the multiple data channels in a distributed fashion. Access to the data channels is coordinated among the nodes by communicating message requests and corresponding replies on the separate reservation channel.
Any node in the network can also use the CSMA/CA technique described above to determine its distance or range to nodes at known locations, to thus enable a node to determine its own location. Specifically, as can be appreciated by one skilled in the art, any node can determine its location by determining its range from three known reference points. A node can therefore use the CSMA/CA technique to determine its ranges from three nodes at known locations, and can thus use these range values to calculate its own geographic location.
An example of this type of ranging is described in U.S. patent application Ser. No. 09/365,702, referenced above. In determining its range to a destination node, a source node transmits an RTS-T message to the intended destination node which, if available, responds with a time of arrival (TOA) message. To perform the ranging, the controller of the source node starts a timer immediately following transmission of the RTS-T message, and stops the timer when synchronization to the TOA message is obtained. The value of the timer represents the aggregate time equal to twice the propagation delay between the source and destination nodes, plus the fixed delay at the destination node that occurs between its receipt of the RTS-T message and transmission of the first symbol of the TOA message, and the duration of time for the source node to complete the synchronization sequence. The source node can perform the ranging during a desired number of RTS-T and TOA exchanges (e.g., ten RTS-T and TOA exchanges), and can average the ranged values to obtain a more accurate ranging value.
Although the ranging technique described above is suitable for obtaining an accurate ranging measurement, the technique can have several drawbacks. For example, the 4096 bit TOA sequence unnecessarily consumes precious bandwidth on the reservation channel. That is, assuming that the RTS-T waveform is 320 microseconds, the TOA reply is much greater in length in order to provide position information (e.g., GPS coordinates), delay calibration information, curve fitting results, and so on. If this TOA reply is transmitted on the reservation channel, the transmission reduces the amount of time available to make data channel reservations, and thus idles these data channels when they could be delivering messages. On the other hand, if the TOA reply is transmitted on a data channel, the two nodes involved in transmitting and receiving the TOA message are unable to monitor the reservation channel during this period, and thus miss information transmitted over the reservation channel pertaining to the channel reservations that are made while these node are tuned away from the reservation channel.
Furthermore, the turnaround time caused by the delay calibration (if required) and the curve fitting results is excessive and directly impacts the reservation channel. For example, calibration of a node thru an internal loopback removes the node from being tuned to the reservation channel, which can result in channel collisions, loss of routing updates, and the node""s failure to respond to requests from other nodes. All of these occurrences can have negative performance implications on the network. Also, by requiring the reference node (i.e., the node being ranged) to perform the calibration while the ranging node is waiting for a response unnecessarily ties up the reservation channel which, in a multi-hop ad-hoc network, could result in a lost channel reservation attempt by another node due to both the near/far and the hidden terminal problems as can be appreciated by one skilled in the art.
It can be further noted that when the transceivers of the source and destination nodes perform any ranging transmissions (e.g., TOA transmissions) over a data channel when performing the ranging operations described above, or when exchanging any information messages, they are incapable of monitoring the information being transmitted from other nodes over the reservation channel. Therefore, during these periods, the source and destination nodes can miss messages pertaining to channel reservations that are being made by other nodes in the network.
In order to eliminate this deficiency, each node in the network can include a secondary receiver in addition to its primary receiver. The secondary receiver permits each node to continuously monitor the reservation channel, even when transmitting or receiving a message on one of the data channels. While monitoring the reservation channel, the nodes store the channel reservations that have been accomplished and avoid use of those channels until the reservations expire. The dedication of the second receiver eliminates the loss of reservation knowledge that occurs with the use of a single receiver for both the reservation and data transfer mechanisms, which is a serious limitation of conventional CSMA/CA schemes. By transmitting requests for channel access on a separate reservation channel and dedicating a receiver to receive and respond to such requests, transmission of information messages on the multiple parallel data channels can be coordinated among the nodes, collisions between request messages and information messages are eliminated, and collisions between the short request messages transmitted on the reservation channel are dramatically reduced. Further details of these features are set forth in U.S. patent application Ser. No. 09/705,588, referenced above.
Although the addition of a secondary receiver to the nodes is effective in enabling the nodes to monitor the reservation channel while transmitting and receiving data over a data channel, it is not always possible or desirable to add a second receiver to a node. For example, it may be preferable to avoid employing a second receiver in a mobile user terminal, such as a mobile telephony device, in order to keep the device as compact as possible. It may also be desirable to avoid employing a second receiver in certain types of nodes in order to reduce their overall cost and manufacturing complexity.
In addition, as discussed above, in order for a source node to retrieve an information message (MSG) from a destination node, a total of 8 transmissions (i.e., two series of RTS/CTS/MSG/ACK messages) must occur between the source and destination nodes. It is noted that when the MSG and ACK messages are being transmitted and received by the source and destination nodes, the primary receivers of those nodes are tuned to a data channel, and it is necessary for the secondary receivers to monitor the reservation channel. However, it would be desirable for the source node to be capable of retrieving an information message from the destination node on the reservation channel, to thus eliminate or at least minimize the need for a second transceiver.
Accordingly, a need exists for a system and method that is capable of reducing the number of transmissions required for a node to range other nodes while also enabling a node to continuously monitor the reservation channel during the ranging process, without the use of a second receiver. A need also exists for a system and method for enabling a source node to retrieve an information message from a destination node on a data channel with a reduced number of transmissions.
An object of the present invention is to provide a system and method for reducing the number of transmissions required for a node in a wireless communications network to range other nodes in the network, while also enabling the node to continuously monitor the reservation channel during the ranging process without the use of a second receiver, to enable the node to more efficiently determine its geographic location.
Another object of the present invention it to provide a system and method which enables a node in a wireless communications network to substantially reduce the amount of time the node is unable to monitor the reservation channel, while receiving information messages from other nodes in the network.
These and other objects of the present invention are substantially achieved by providing a communication node which is adapted for use in a wireless communications network and comprises a transceiver and a controller. The transceiver is adapted to transmit and receive messages to and from other nodes in the network over at least one of a plurality of shared data channels, and is tunable to a reservation channel to monitor channel access reservation messages transmitted by the other nodes in said network. The controller is adapted to control the-transceiver to transmit a range request message to at least one other of the nodes in the network over the reservation channel and to receive a range reply message from at least one other node over the reservation channel in response to the range request message, to enable the controller to determine a distance of the communication node from the other node based on the range request message and the range reply message. Specifically, the range request message is configured so as to indicate to the node being ranged, as well as any other node within the radio frequency (RF) range of the node performing the ranging, that the transceivers of the ranging and ranged nodes are not transitioning to a data channel. The controller can determine the distance by measuring a duration of time that elapses between a first moment when a portion of the range request message is transmitted by the transceiver and a second moment when a portion of the range reply message is received by the transceiver. The controller is further adapted to control the transceiver to receive an information message that has been transmitted over a reserved data channel by the other node substantially immediately after the other node transmitted the range reply message.